Plasma processes are widely used in semiconductor manufacturing, for example, to implant wafers with various dopants, to deposit or to etch thin films. In order to achieve predictable and repeatable process results, it is critical to closely monitor and control the plasma characteristics. For example, studies of plasma doping (PLAD) processes have shown that ion composition of a plasma may be a critical piece of information that determines dopant species, dopant depth profiles, process-related contamination, etc. The ion composition changes with PLAD process parameters such as gas ratio, total gas pressure, and discharge power. The ion composition can also change significantly depending on the conditioning status of a plasma chamber. Therefore, it is important to know the ion composition during a PLAD process, preferably in situ and in real-time, in order to achieve repeatable and predictable process results.
Existing plasma tools often lack the capability of providing detailed real-time information (e.g., ion composition) of a plasma. In a typical PLAD process, for example, a plasma is controlled by monitoring an implant dose based on a Faraday cup current. However, a Faraday cup is just a total charge counter which does not distinguish different charged particles or otherwise offer any insight of the plasma. Although in-situ mass analysis has been employed in some traditional beam-line ion implantation systems, it has typically been avoided in plasma-based ion implantation systems in order to achieve a high throughput.
In addition, conventional ion sensors, such as commercial mass/energy analyzers and quadrapole mass spectrometers, are often too bulky and/or too intrusive to implement in production tools. Large ion sensors tend to perturb a plasma under measurement and therefore distort process results. Furthermore, the size and weight of conventional ion sensors often limit their deployment options in a semiconductor process tool. Furthermore, in pulsed plasma processing wherein a plasma alternates between on and off states, time-resolved measurements of the plasma are often required. However, few existing ion sensors provide the capability of time-resolved measurements.
In view of the foregoing, it would be desirable to provide a technique for monitoring ion species which overcomes the above-described inadequacies and shortcomings.